1. Field of the Invention
The present invention relates to a multi-phase stirrer for stirring the molten core around or along a cast string or slab coming from a continuous casting machine and arranged around or along the cast slab.
2. Prior Art
Stirring of a cast slab can be achieved by accelerating the melt along or across the cast slab, and also, of course, at any angle between these directions. Normally, a straight stirrer is used or the melt is accelerated by a rotating field using a round stirrer of the motor stator type.
In a continuous casting machine molten metal is poured into a water-cooled mold, the solidified slab being drawn out downwards through the mold. When the slab leaves the mold, it consists of a thinly solidified shell and a liquid core. After the cast slab has left the mold, it is normally sprayed with water in a secondary cooling zone, the solidifying shell then increasing in thickness until the whole cast slab has solidified. During the solidification process, it is desirable for the still liquid core to be stirred, since this, among other things, counteracts segregation phenomena and formation of cracks, as well as formations of large pores and voids, and among other things so-called pipes.
A known device for achieving stirring is a so-called round stirrer, in which the melt is rotated around the center line of the slab in a plane perpendicular to the withdrawing or transportation direction of the slab. This rotation is effected by a rotary magnetic field in the same plane. The round stirrer is often designed in principle as a two-pole, three-phase motor stator through which the slab passes.
However, the continuous casting machine is often mechanically built up from a framework of tubes. Normally, support rolls are attached in these tubes and control the slab. The tubes and support rolls are mechanically divided into a number of segments which are exchanged and replaced by spare segments during maintenance. This procedure considerably facilitates the mechanical maintenance, but the framework constitutes an obstacle to the location of the stirrers for the stirring purpose just mentioned.
The round stirrer can be positioned either between the tubes of the machine and the slab, or in such a way that it surrounds both the tubes and the slab. In the firstmentioned case the stirrer has to be very small and it will also be located so close to the slab that this will make water-cooling difficult.
In the second case, the stirrer has to be positioned so far from the slab that its effect is strongly reduced. In both cases problems will arise when the segment, where the stirrer is placed, is to be replaced. These types of stirrers are also difficult to adjust to different cross-sectional dimensions of the slab, and still more difficult to adjust to slabs with non-square cross-sections.